View source for 31-bit computing

{{Short description|Computer architecture bit width}}
{{Computer architecture bit widths}}
{{For|how 31/32-bit applies to networking and 31-bit subnet masks|Subnetwork}}
{{More citations needed|date=May 2023}}
In [[computer architecture]], '''31-bit''' [[integer (computer science)|integer]]s, [[memory address]]es, or other [[data (computer science)|data]] units are those that are 31 [[bit]]s wide. 

In 1983, [[IBM]] introduced 31-bit addressing in the [[System/370-XA]] [[Mainframe computer|mainframe]] architecture as an upgrade to the [[24-bit computing|24-bit]] physical and virtual,<ref name=Hist24.31plus>{{cite web
|url=https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zconcepts/zconcepts_102.htm
|title=A brief history of virtual storage and 64-bit addressability|website=[[IBM]] }}</ref> and transitional 24-bit-virtual/[[26-bit computing|26-bit]] physical,<ref>"with transitional support for 26-bit"</ref><ref name=A26>{{cite web
|title=Development and attributes of z/Architecture"
|url=https://www.cl.cam.ac.uk/teaching/0607/CompArch/ibm-z-plambeck.pdf
|author=KE Plambeck  |date=2002}}</ref> addressing in [[System/370]].<ref name=DataM31.tr>{{cite magazine 
|title=XA: The View From The Trenches (pp.122-136)
|magazine=Datamation 
|date=May 1983 |author=Robert T. Fertig}}</ref><ref name=DataM31.WP>{{cite magazine |title=XA: The View From White Plains (pp.139–152)
|magazine=Datamation |date=May 1983 |author=Ronald L. Bond}}</ref> This enhancement allowed address spaces to be 128 times larger, permitting programs to address memory above 16&nbsp;[[Megabyte|MB]] (referred to as "above the line").<ref>"...to run in the 31-bit area above the line,... {{cite magazine
|magazine=Computerworld |date=October 27, 1986  
|page=13 |url=https://books.google.com/books?id=nI091j0HhhwC |title=Rewriting to run in 31 bit area}}</ref><ref name=Hist24.31plus/> Support for [[COBOL]], [[FORTRAN]] and later on [[Linux on IBM Z|Linux/390]] was included.

In the early 1980s, the [[Motorola 68012]] was introduced; it had [[32-bit computing|32-bit]] data and address registers, as did the [[Motorola 68010]], but instead of providing the lower 24 bits of an address, it did so for all but bit 30 on the address pins.<ref>{{cite manual
 |       title = MC68010/MC68012 16-/32-Bit Virtual Memory Microprocessors
 |          id = ADI942R2
 |        date = May 1985
 |       pages = 4-1-4-2
 |     section = 4.1 SIGNAL DESCRIPTION 
 | section-url = http://bitsavers.org/components/motorola/68000/MC68010_68012_Data_Sheet_May85.pdf#page=30
 |         url = http://bitsavers.org/components/motorola/68000/MC68010_68012_Data_Sheet_May85.pdf
 |        work = Advance Information
 | publisher = Motorola Semiconductor
 }}
</ref>

==31-bit computer==
[[File:LGP-30.jc.jpg|thumb|Librascope LGP-30 supported 31-bit fixed-point numbers]]

The [[LGP-30|Librascope LGP-30]] was an early off-the-shelf computer. The LGP-30 was first manufactured in 1956,<ref>{{Cite web |url=http://www.librascopememories.com/Librascope_Memories/1950_-_1959.html |title=1950-1959 Librazettes |website=www.librascopememories.com |language=en |quote=Librazette: July, 1956 – ''Royal Precision Plans – LGP-30 Promotion''; November, 1956 – ''LGP-30, Flow Computer Spearhead – Commercial Sales and Production – They're In Production'' and ''Paul Coates Will Feature LGP-30 on Dec. TVshows'' |access-date=2018-03-19 |archive-url=https://web.archive.org/web/20141227210956/http://www.librascopememories.com/Librascope_Memories/1950_-_1959.html |archive-date=2014-12-27 |url-status=dead }}{{verification needed|date=June 2023|reason=This cite is for an index of "Librazettes" and the individual links fail.}}</ref> at a retail price of $47,000,{{citation needed|date=June 2023}} {{Inflation|US|47000|1956|r=-4|fmt=eq}}.{{Inflation/fn|US}}

It was a [[Binary number|binary]], 31-bit word computer with a 4096-word [[drum memory]]. There were 32 bit locations per drum word, but only 31 were used, permitting a "restoration of magnetic flux in the head" at the 32nd bit time. The number of [[vacuum tube]]s was minimized by using solid-state [[diode logic]], a [[bit-serial architecture]] and multiple use of each of its 15 flip-flops.

The LGP-30 was commonly referred to as a desk computer. Its height, width, and depth, excluding the typewriter shelf, was 33 by 44 by 26 inches (84 by 112 by 66 cm). It weighed about 800 pounds (360 kg), and was mounted on sturdy casters which facilitated moving the unit.

==IBM mainframes with 31-bit addressing==
In the [[System/360]], other than the [[IBM System/360 Model 67|360/67]], and early [[System/370]] architectures, the [[general-purpose register]]s were 32&nbsp;bits wide, the machine did 32-bit arithmetic operations, and addresses were always stored in 32-bit words, so the architecture was considered [[32-bit computing|32-bit]], but the machines ignored the top 8 bits of the address resulting in [[24-bit computing|24-bit]] addressing.

With the [[IBM System/370-XA|System/370-XA]] architecture and the [[IBM Enterprise Systems Architecture]], in addition to a 24-bit addressing mode for compatibility with older applications, there is a 31-bit addressing mode, in which only the high order bit (bit 0) in the word is ignored for addressing. An exception is that mode-switching instructions also use bit 0. There were at least two reasons that IBM did not implement the 32-bit addressing of the 360/67
# The loop control instructions '''BXH''' and '''BXLE''' did signed comparisons.
# Much of the existing software used bit 0 as an end-of-list indicator.<ref>"... the high order bit in the last fullword must be set to one to indicate the end of the list." {{cite web
|url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.ieaa400/wait.htm
|title=WAIT — Wait for one or more events|website=[[IBM]] }}</ref>

The 64-bit [[z/Architecture]] also supports 24-bit and 31-bit addressing modes for compatibility with older applications.

==Ruby and Smalltalk==
Interpreters for Ruby and Smalltalk languages use the lowest bit to tell whether a value is an unboxed integer or not. This means that on 32-bit machines (or [[16-bit computing|16-bit]] machines with 32-bit pointers), 31-bit integers are unboxed. In case of overflow, the result is fit into a boxed object, which means that it has to be allocated and garbage-collected. Thus if you have 32-bit values that don't fit in the 31-bit signed format, they will be very inefficient in those interpreters. Same goes with 63-bit unboxed integers on 64-bit computers. Similar designs may be found in LISP and some of the other languages whose variables can take values of any type. In some cases, there was hardware support for this kind of design: see [[Tagged architecture]] and [[Lisp machine]].

==References==
{{reflist}}

{{CPU technologies}}

[[Category:Data unit]]